In image formation based on an electrophotographic process, a latent electrostatic image is formed on a latent electrostatic image bearing member containing a photoconductive material or the like, a charged toner is made adhere onto the latent electrostatic image to form a toner image, and then the toner image is transferred onto a recording medium, and fixed on the recording medium to be an output image. In recent years, there have been rapid developments from monochrome image technologies toward full color image technologies of copiers and printers using electrophotographic processes, and the market of full color image technologies increasingly tends to expand.
Typically, in color image formation based on a full color electrophotographic process, all colors are reproduced by superimposing three primary color toners of yellow, magenta, and cyan or four color toners with black color toner added to the three primary colors. Therefore, to obtain a full color image having excellence in color-reproducibility and color vividness, the surface of the fixed toner image must be smoothed and evened to some extent to reduce scattering of light. For this reason, there were so many conventional types of full color copiers to or the like which have a middle level of image glossiness to high level image glossiness of 10% to 50%.
Typically, as a method of fixing a dry toner image on a recording medium, a contact-heating fixing method is heavily used in which a roller or belt having a smooth surface is heated and the surface is press-contacted with a toner. Such a fixing method achieves high thermal efficiency, enables high-speed fixation, and enables imparting glossiness and transparency to color toners, but on the other hand, a so-called “offset phenomenon” is liable to take place in which a part of toner image is attached to a surface of a fixing roller and then transferred onto another image, because it is necessary that the surface of a heat-fixing member be brought in contact with a toner in a molten state under application of pressure and then be separated from the heat-fixing member.
For the purpose of preventing the offset phenomenon, a method is typically employed in which the surface of a fixing roller is formed of a silicone rubber, a fluororesin or the like, and further, an oil for preventing adhesion of toner, such as a silicone oil, is applied on the surface of the fixing roller. This method is extremely effective in preventing toner offset, however, it requires a device for supplying the oil, and has a problem that the fixing unit becomes large in size.
For this reason, in monochrome image formation, there is a tendency to employ an oil-less system where no oil is applied to a fixing roller or a system that requires only a small amount of oil applied to a fixing roller, by using a toner which has high viscoelasticity in the molten state and contains a releasing agent so that the molten toner does not internally fracture.
Meanwhile, also in full color image formation, similarly to monochrome image formation, there is a tendency to employ oil-less systems for the purpose of achieving downsizing and structural simplification of fixing devices. In full color image formation, in order to smooth the surface of a fixed toner image, it is however necessary to reduce the viscoelasticity of toner in the molten state, and thus an offset phenomenon is more likely to occur than formation of monochrome images which have no glossiness, making it difficult to employ oil-less systems. When a toner containing a releasing agent is used, the transferability of the toner onto a recording medium degrades due to increased adhesiveness of the toner. Further, toner filming occurs, and the chargeability of the toner degrades, causing a degradation of the durability.
In the meanwhile, as a carrier, there has been known a carrier whose surface is coated with a silicone resin, for the purpose of preventing toner filming, forming a surface with a uniform thickness, preventing oxidation of the surface as well as a reduction in moisture sensitivity, prolonging the life of developers, preventing adhesion of toner onto a photoconductor, protecting the photoconductor from scratches and abrasion, controlling the charge polarity, and adjusting the charge amount.
For example, Patent Literature 1 discloses a carrier in which surface of core material particles are coated around with a silicone resin containing an organic titanium catalyst.
Patent Literature 2 discloses a carrier in which surfaces of core material particles are coated with a coating agent containing, as main components, organopolysiloxane, organosilane, and a coatable composition which is composed of a curing catalyst containing at least one selected from the group consisting of titanium, tin, zinc, cobalt, iron, an aluminum compound, and amines.
Further, Patent Literature 3 discloses a carrier in which surfaces of core material particles are coated with a quaternary aluminum catalyst, a silicone resin containing to a titanium catalyst or a modified silicone resin.
However, the techniques disclosed in the prior art cause problems with blocking and a degradation of durability of the carrier.
[Citation List]
[Patent Literature]                [PTL 1] Japanese Patent Application Laid-Open (JP-A) No. 2001-92189        [PTL 2] Japanese Patent Application Laid-Open (JP-A) No. 06-222621        [PTL 3] Japanese Patent Application Laid-Open (JP-A) No. 2006-337828        